1. Field of the Invention
This invention relates to the field of electrical power assist steering systems for automotive vehicles and more specifically to the area of increasing the output of such systems to enable them to be used in heavy duty vehicles.
2. Description of the Prior Art
Generally, power assisted steering systems for automotive vehicles are classified as either hydraulic power assisted steering (“HPAS”) or electrical power assisted steering (“EPAS”). A pure HPAS system is the type in which a fixed displacement hydraulic pump is directly driven from the vehicle engine to supply pressurized fluid through a hydro-mechanical control valve to a steering gear where the pressure is differentially applied to a piston or other steering actuator mechanism on the steering gear. A pure EPAS system is the type in which an electrically powered motor is controlled by sensor reactive circuitry to apply assisting torque directly to the steering gear or other steering actuator mechanism. All control to the steering gear in a pure EPAS system is electro-mechanical in nature and no hydraulic systems are involved. EPAS systems generally provide for a greater use of sensors throughout the vehicle and allow steering assist to be adjusted in accordance with driver input through the steering wheel and other factors, such as speed of the vehicle, rate of steering wheel torque and many other variables that were not available in pure HPAS systems.
FIG. 1 illustrates a block diagram of a typical prior art EPAS system as employed in an automotive vehicle. In such a system, a control module 100 contains an electrical controller 110 which feeds an output control signal to a power electronics module 120, which, in turn, supplies electrical power on line 140 to an actuating torque motor 150. Torque motor 150 contains a gear 160 on its output shaft which is engaged directly or through a gear mechanism with a steering gear 212 within a rack housing 210. In this depiction, the driver input is represented as a steering wheel 250. A torque transducer 240 is located on the steering wheel shaft to provide informational data to the controller 110. Such informational data includes the torque being applied by the driver to the steering wheel. Also, it may sense the position of the steering wheel as well as its distance from center. Steered vehicle wheels are represented as output mass 260 connected to steering gear 212. Controller 110 is connected to receive input data from the vehicle such as vehicle speed, steering wheel position and steering wheel input torque. Based upon such data, controller 110 utilizes an algorithm to determine how much assistance torque to apply to the steering gear 212 through torque motor 150 and its gear 160.
It is highly desired to employ EPAS systems in heavy duty vehicles. However, when using EPAS systems with a conventional 12 volt DC electrical power system, there are practical limitations that must be overcome or accommodated. Due to the larger steering loads encountered by placing such systems in heavier vehicles, there would be a requirement for higher capacity power electronics and larger motor components. The increased cost of these components makes EPAS systems uncompetitive with HPAS systems in high load applications. Even when cost is ignored, the maximum current available from the vehicle electrical system is a real-life barrier to implementation. As a practical alternative, several variations of hybrid systems have been developed in which a hydraulic-mechanical link of an HPAS system is maintained, to some degree, while one or more control functions applied to a hydraulic assist are electrically controlled to provide greater response to various vehicle data.
There continues to be a need for an improved EPAS system or some version of a hybrid EPAS system in which steering assist could be applied to heavy duty vehicles, while minimizing the detrimental effects of the system on the operating efficiencies of the vehicle.